This invention relates to improved incremental path measuring devices which measure length or angle.
A variety of path measuring devices for determining the relative position of two objects have been described in both direct-light and transillumination embodiments in many publications. It is known that in measuring devices employing incremental measuring arrangements, the optimal separation between the scanning grid of the scanning unit and the measurement grid of the measuring scale of the measuring device depends directly on the chosen grid constant of the measurement grid or division. This optimal scanning separation is subject to very close tolerances. This is because the magnitude of the scanning signal generated by photosensitive components of the scanning unit depends directly on the intensity of the light modulation arising from relative movement between the scanning and measurement grids, and the light modulation is strongly influenced by the scanning separation.
It is known that the intensity of the modulated scanning signal is greatest when the scanning separation has the value of zero, and the scanning grid and the measurement grid of the measurement apparatus lie in a single plane; only then is there an exact coincidence of the respective lines and gaps of the two grids. As the scanning separation increases, the intensity of the modulated scanning signals decreases.
A direct sliding contact of the scanning plate (which defines the scanning grid) upon the measurement scale (which defines the measurement grid) is impractical because of the high potential for damage to these components. The interposition of optical elements for the imaging of one grid on the other is undesirable, at least in the case of small and medium grid constants, because the cost of the device would be increased substantially.
In practice, therefore, a certain optimal scanning separation is generally chosen for the selected grid constant and very close tolerances are employed for this scanning separation. For well-known reasons, light intensity at the photosensitive elements of the scanning unit increases when the scanning separation decreases and, conversely, decreases when the scanning separation is increased. This holds as long as the scanning separation remains close to the value zero.
Such an increase or decrease of light intensity caused by distance variations is acceptable, however, only in a certain narrow range, in view of the necessary signal amplification. This is because amplifiers of the type widely used for this special application function reliably only if the input signals to these amplifiers do not drop below a certain minimum value and do not exceed a certain maximum value. In the event an input signal were to deviate from the admissible drive range of the amplifier, distortions of the output signal of the amplifier would occur. For example, with excessively low signals, the susceptibility to problems related to internal noise rises.
Various measures have been proposed to alter the aforementioned relations of scanning distance and light flux or intensity in a modulated scanning system in order to improve photoelectric measuring devices.
U.S. Pat. No. 3,812,352, for example, suggests that the scanning separation be selected according to the formula Z.sub.1 =nS.sub.1 S.sub.2 /.lambda., in which Z.sub.1 is the scanning separation or distance, S.sub.1 is the grid constant of the first grid, S.sub.2 is the grid constant of the second grid, .lambda. is the light wavelength, and n is a positive whole number. If n is chosen to have a value of 1 in this formula, then for the grid constants given in the above-identified patent the relations shown there in FIGS. 5 and 6 between light intensity and scanning distance are achieved. With grids of differing grid constants, however, this approach is applicable only for the specific conditions presented therein, i.e., only for measuring arrangements which operate in the direct-light mode and which employ fine grid constants (e.g., 25 microns).
Another approach to the selection of favorable conditions for scanning tolerances is suggested in German Pat. No. 25 10 273. The vignetting proposed there of the photosensitive components leads to the desired result, but only in the case of grid constants that are greater than 100 microns, and in measuring systems which, as indicated therein, operate in the direct-light mode.
These teachings, however, are practically inapplicable in measuring devices that are intended for medium grid constants between 25 and 100 microns and which operate either as direct-light or as transilluminating measuring devices.